IFR Fix: Finding zero

Don’t peek, just take an educated guess. Then get a weather briefing and check your estimate.

Bonus points if you nailed it, because summer isn’t a time when the 0-degrees-Celsius level is a piston pilot’s first thought with thunderstorms and density-altitude concerns competing for attention.

The freezing level’s seasonal trends can be tracked within certain limits. In late August, a pilot at New Hampshire’s latitude would observe a median freezing level near 14,000 feet, according to a 12-month analysis by the online North American Freezing Level Tracker from the Western Regional Climate Centers. Resetting the parameters showed that the freezing level lurked around 15,000 feet over Maryland, as depicted by the graph’s undulating line of values over 12 months.

After the weather briefing tells you where the freezing level is forecast for your flight, you must then decide whether the big weather picture of clouds and precip permits safe crossing of the 0-degrees-Celsius line.

If not, what’s the plan for avoiding it? Will the plan hold up over time?

A Lancair pilot cautiously planned a less-than-direct route to Manassas, Va., to avoid forecast precipitation. The 7,000-foot cruise altitude, in clear air above clouds, was designed to keep the flight beneath a forecast 8,000-foot freezing level.

As the flight proceeded, the outside air temperature dropped from 40 degrees Fahrenheit to 34 degrees, then to 32, then to 30, even after descent to 5,000 feet, as related in an Aviation Safety Reporting System narrative submitted without evident need to exculpate an incident or violation.

"I started feeling really uncomfortable with the situation (IMC in below freezing temperatures) and couldn't understand how I found myself in the position," the pilot wrote.

Perhaps the report revealed a rote expectation that the air would warm as the aircraft descended. That would be true with a standard lapse rate, but not after formation of a temperature inversion.

A GPS approach concluded matters. The aircraft broke out at 900 feet, having avoided the sweet spot for icing.

Still the pilot believed it would have been wiser to divert visually at the first sign of temperatures aloft misbehaving. The narrative captured the emotion any pilot might experience after being caught off guard by changing conditions.

"I was happy to make it safely on the ground, but I had a sick feeling in my gut," the pilot wrote.